Acoustic wave filter, multiplexer, radio frequency front-end circuit, and communication device

1. An acoustic wave filter comprising: a series-arm circuit disposed on a path that connects a first input/output terminal and a second input/output terminal of the acoustic wave filter; and a parallel-arm circuit connected to a node on the path and a ground, wherein at least one of the parallel-arm circuit and the series-arm circuit includes: a first acoustic wave resonator; and a first comb-shaped capacitor connected in parallel or series to the first acoustic wave resonator, the first comb-shaped capacitor includes a comb-shaped electrode that includes electrode fingers, a frequency at which impedance of the first comb-shaped capacitor has a local maximum value is located outside a passband of the acoustic wave filter, and under a condition that a proportion of a width of each of the electrode fingers to a value is defined as an electrode finger duty ratio, the value being a sum of the width and a space between adjacent electrode fingers, the comb-shaped electrode has (i) at least two different electrode finger pitches, (ii) at least two different electrode finger duty ratios, or (iii) at least two different electrode finger pitches and at least two different electrode finger duty ratios.

2. The acoustic wave filter according to claim 1 , wherein the parallel-arm circuit further includes a first switch connected in parallel to the first comb-shaped capacitor, and a first switch circuit in which the first comb-shaped capacitor and the first switch are connected in parallel is connected in series to the first acoustic wave resonator.

3. The acoustic wave filter according to claim 2 , wherein the parallel-arm circuit further includes a second acoustic wave resonator, wherein the second acoustic wave resonator is connected in parallel to a circuit in which the first acoustic wave resonator and the first switch circuit are connected in series to each other, has a resonant frequency different from a resonant frequency of the first acoustic wave resonator, and has an antiresonant frequency different from an antiresonant frequency of the first acoustic wave resonator.

4. The acoustic wave filter according to claim 3 , wherein the parallel-arm circuit further includes a second switch circuit connected in series to the second acoustic wave resonator, a circuit in which the second acoustic wave resonator and the second switch circuit are connected in series to each other is connected in parallel to a circuit in which the first acoustic wave resonator and the first switch circuit are connected in series to each other, the second switch circuit includes: a second comb-shaped capacitor; and a second switch connected in parallel to the second comb-shaped capacitor, wherein a frequency at which impedance of the second comb-shaped capacitor has a local maximum value is located outside the passband of the acoustic wave filter, the second comb-shaped capacitor includes a comb-shaped electrode that includes electrode fingers, and the comb-shaped electrode included in the second comb-shaped capacitor has (i) at least two different electrode finger pitches, (ii) at least two different electrode finger duty ratios, or (iii) at least two different electrode finger pitches and at least two different electrode finger duty ratios.

5. The acoustic wave filter according to claim 1 , wherein the series-arm circuit further includes a first switch connected in series to the first comb-shaped capacitor, and a circuit in which the first comb-shaped capacitor and the first switch are connected in series is connected in parallel to the first acoustic wave resonator.

6. The acoustic wave filter according to claim 1 , wherein the parallel-arm circuit further includes a second switch connected in parallel to the first comb-shaped capacitor, a circuit in which the first comb-shaped capacitor and the second switch are connected in parallel is included in a first switch circuit, and the first switch circuit is connected in series to the first acoustic wave resonator.

7. The acoustic wave filter according to claim 1 , wherein the first acoustic wave resonator and the first comb-shaped capacitor are connected in parallel, the comb-shaped electrode includes electrode fingers spaced at a first electrode finger pitch, and other electrode fingers spaced at a second electrode finger pitch greater than the first electrode finger pitch, and a pitch difference is at least 0.2%, the pitch difference being a difference between the first electrode finger pitch and the second electrode finger pitch, divided by the first electrode finger pitch.

8. The acoustic wave filter according to claim 7 , wherein out of the electrode fingers included in the comb-shaped electrode, a total number of electrode fingers spaced at the first electrode finger pitch is in a range from 10% to 80% of a total number of the electrode fingers included in the comb-shaped electrode.

9. The acoustic wave filter according to claim 7 , wherein out of the electrode fingers included in the comb-shaped electrode, a total number of electrode fingers spaced at the first electrode finger pitch is in a range from 20% to 50% of a total number of the electrode fingers included in the comb-shaped electrode.

10. The acoustic wave filter according to claim 1 , wherein the first acoustic wave resonator and the first comb-shaped capacitor are connected in series, the comb-shaped electrode includes electrode fingers spaced at a first electrode finger pitch, and other electrode fingers spaced at greater than the first electrode finger pitch, and a pitch difference is in a range from 0.2% to 6.3%, the pitch difference being a difference between the first electrode finger pitch and the second electrode finger pitch, divided by the second electrode finger pitch.

11. The acoustic wave filter according to claim 1 , wherein the first acoustic wave resonator and the first comb-shaped capacitor are connected in series, the comb-shaped electrode includes electrode fingers spaced at a first electrode finger pitch, and other electrode fingers spaced at a second electrode finger pitch greater than the first electrode finger pitch, and out of the electrode fingers included in the comb-shaped electrode, a total number of electrode fingers spaced at the first electrode finger pitch is in a range from 10% to 65% of a total number of the electrode fingers included in the comb-shaped electrode.

12. The acoustic wave filter according to claim 1 , wherein the first acoustic wave resonator and the first comb-shaped capacitor are connected in series, the comb-shaped electrode includes electrode fingers spaced at a first electrode finger pitch, and other electrode fingers spaced at a second electrode finger pitch greater than the first electrode finger pitch, and a pitch difference is at least 0.2%, the pitch difference being a difference between the first electrode finger pitch and the second electrode finger pitch, divided by the first electrode finger pitch.

13. The acoustic wave filter according to claim 1 , wherein the first acoustic wave resonator and the first comb-shaped capacitor are connected in series, the comb-shaped electrode includes electrode fingers spaced at a first electrode finger pitch, and other electrode fingers spaced at a second electrode finger pitch greater than the first electrode finger pitch, and out of the electrode fingers included in the comb-shaped electrode, a total number of electrode fingers spaced at the first electrode finger pitch is in a range from 10% to 90% of a total number of the electrode fingers included in the comb-shaped electrode.

14. The acoustic wave filter according to claim 1 , wherein the comb-shaped electrode includes electrode fingers spaced at a first electrode finger pitch, and other electrode fingers spaced at a second electrode finger pitch greater than the first electrode finger pitch, and out of the electrode fingers, electrode fingers spaced at the first electrode finger pitch are thinner than electrode fingers spaced at the second electrode finger pitch.

15. The acoustic wave filter according to claim 1 , wherein the first acoustic wave resonator includes an interdigital transducer (IDT) electrode that is formed on a piezoelectric substrate and includes electrode fingers, the comb-shaped electrode is formed on the piezoelectric substrate, and the at least two different finger pitches of the comb-shaped electrode are each shorter than an electrode finger pitch of the IDT electrode.

16. The acoustic wave filter according to claim 15 , wherein the electrode fingers included in the comb-shaped electrode are thinner than electrode fingers included in the first acoustic wave resonator.

17. The acoustic wave filter according to claim 15 , wherein the electrode fingers included in the comb-shaped electrode have a duty ratio higher than a duty ratio of the electrode fingers included in the IDT electrode.

18. A multiplexer, comprising: a plurality of filters at least one of which has a same structure as a structure of the acoustic wave filter according to claim 1 , wherein one of the first input/output terminal and the second input/output terminal for each of the plurality of filters is directly or indirectly connected to a common terminal.

19. A radio frequency front-end circuit, comprising: the multiplexer according to claim 18 ; and an amplifier circuit directly or indirectly connected to the multiplexer.

20. A communication device, comprising: a radio frequency (RF) signal processing circuit that processes a radio frequency signal to be transmitted by an antenna and a radio frequency signal received by the antenna; and the radio frequency front-end circuit according to claim 19 that conveys the radio frequency signals between the antenna and the RF signal processing circuit.